THERE IS now abundant scientific proof that HIV can spread between the sexes during vaginal intercourse. Scientists also have evidence for 'second-generation' transmission from one heterosexual to another and then a third.
Studies of hundreds of stable heterosexual couples have found that over a period of several years there is about a 20 per cent risk of infected men passing the virus to their female partners.
The risk of woman-to-man transmission is about half that.
Detailed questionnaires of the sexual activities of the couples revealed that anal sex and sex during menstruation were not essential for transmission of HIV.
In one study of 563 European couples, '40 out of 82 infected women never practised anal sex and 8 out of 19 infected men never had intercourse during their partners' menses'.
Scientists have used such data to assess the infectiousness of HIV. They conclude that an uninfected woman has about a 1 in 1,000 risk of catching the virus from having sex once with an infected man - the risk is about half for transmission from women to men.
Although this shows that HIV is not as infectious as many other sexually transmitted diseases, it does not offer grounds for complacency. The dangerous and often overlooked aspect of HIV is that infected people remain infectious for many years before showing the first signs of Aids.
Professor Roy Anderson, of Imperial College in London, who constructs mathematical models of how HIV is likely to spread in years to come, says that a risk of 1 in 1,000 per sex act may not appear worrying.
But what is worrying is that infected heterosexuals can typically have sex twice a week for 10 years without knowing they are HIV-positive. This means the epidemic of Aids, unlike past scourges such as bubonic plague and syphilis, is moving over a period of decades rather than weeks or months.
The ease with which HIV can pass from one couple to another is illustrated by the case of a woman in her twenties, who had passed HIV to her boyfriend. Later in the same year, after that relationship had ended, he passed the virus to his new partner, another woman in her twenties. None of the three had injected drugs.
Scientists who have studied this and other cases of 'second-generation' transmission within the UK suspect that, after a person becomes initially infected, he or she becomes relatively infectious for a period of several weeks as the virus multiplies rapidly within their blood system - what doctors describe as initial 'viraemia'.
A longer period of dormancy, lasting several years, follows. This is apparently matched by a lower level of infectiousness, they suspect. Towards the end of the dormant period, when the virus begins to multiply again prior to the onset of Aids, HIV-positive patients appear to become more infectious again.
Anne Johnson, senior lecturer in epidemiology at University College and Middlesex School of Medicine in London, said that most new HIV infections in heterosexuals were now due to the second-generation spread of HIV and a significant minority - about 10 per cent - were the result of transmission entirely within the UK. This category, she said, showed a 'consistent annual increase'.
Scientists at the Government's Communicable Disease Surveillance Centre in London said that 'anal sex between heterosexuals has contributd very little, if any, to the observed second-generation heterosexual transmission' of HIV.
First-generation transmission - for example from a drug user or bisexual to heterosexual - is still happening in Britain, in spite of couples being aware that one of them is HIV- positive.
Roy Robertson, an Edinburgh GP, who has been following nearly 200 couples where one partner is an HIV- positive drug user, said that he was still seeing uninfected partners becoming HIV-positive.
He saw about six cases a year, he said. One of the youngest was a 17- year-old girl whose boyfriend injected drugs many years ago.
'Many are in long-term relationships where they often do not use condoms and when they see that they are still uninfected, they say the doctors are wrong. Then, when eventually they do get the virus, they are absolutely devastated,' he said.
'They recognise that the whole thing is a mess because the information they are getting from newspapers about the risk of heterosexual Aids is not what their doctors are telling them. As a result, our street cred is pretty low.'